The present invention relates to industrial chemical processes in which a processing chemical initially in the form of a liquid is converted to the vapor state, in which state it is conveyed to a processing station.
In many industrial chemical processes, a processing chemical is stored in a container in a liquid state and the supply of processing chemical in the container is gradually exhausted as the liquid is converted to a vapor and expelled from the container.
According to one technique currently employed for converting such a processing chemical into a vapor, a carrier gas is bubbled through the liquid processing chemical to produce the vapor, which is then conveyed to a processing chamber, normally with the aid of a pump. This technique is currently employed in chemical vapor deposition (CVD) systems employing, as the processing chemical, a liquid chemical such as DMAH, TDMAT, TEOS, etc.
The carrier gas is injected at some point below the surface of the liquid chemical, usually at a point near the bottom of the container in which the chemical is stored. Upon being introduced into the liquid chemical body, the carrier gas forms discrete bubbles which rise to the surface of the liquid body. Each bubble will contain a quantity of the liquid chemical in vapor form. Thus, when the bubbles emerge from the surface of the body of liquid, they provide a quantity of vapor which is then removed from the region above the liquid body. removed from the region above the liquid body.
A number of characteristics of the resulting deposition process are often highly sensitive to any changes in the composition of the liquid chemical or the rate at which it is delivered in vapor form to the process chamber. For example, liquid chemicals employed in processes of the type referred to above can undergo a change in viscosity within their storage container as a result of having become contaminated, or having experienced distillation, precipitation, or polymerization. The resulting change in chemical composition, or quality, can alter the composition or quality of the resulting deposited film in an unacceptable manner. Moreover, changes in the rate at which the chemical is vaporized within its container can cause corresponding changes in the deposition rate, which may also be undesirable.
Measurement of such changes in the liquid chemical could allow such undesirable changes in the resulting deposited film to be anticipated and corrected. However, the nature of many of the chemicals in question, and particularly their high level of corrosivity has often prevented effective measurements from being made by conventional measuring techniques and instruments.
Thus, although it is already known that at least a rough estimate of the viscosity of a liquid chemical can be obtained by visual observation of the bubbles being produced by a carrier gas, such visual observation from an external vantage point usually requires that at least a portion of the liquid chemical container be transparent. However, because of the hazardous nature of many of the chemicals of the type here under consideration, containers which are made in whole or in part of transparent materials may not provide the requisite degree of safety in such applications. Alternatively, the corrosive or sensitive nature of many of these chemicals renders impractical the placement of optical or other instruments inside the container where the chemicals can either corrode the instruments or be contaminated by the instruments.